The present invention relates to an aluminum base for offset printing plates and to a two-stage anodic oxidation process for production of the base. Also disclosed is the offset printing plate itself and the process for producing same.
Bases for offset printing plates are provided, either directly by the user or by the manufacturer of precoated printing plates, with a radiation-sensitive or photosensitive layer (reproduction layer) on one or both sides, with the aid of which layer a printable image is produced by photomechanical means. After production of a printing form from the printing plate, the base carries the image areas which convey ink during subsequent printing and, in the areas which are image-free during subsequent printing (non-image areas), also forms the hydrophilic image background for the lithographic printing process.
Bases for reproduction layers for the production of offset printing plates therefore have to meet the following requirements:
The areas of the radiation-sensitive layer which are relatively more soluble after exposure must be capable of being readily removed from the base without leaving a residue to produce the hydrophilic non-image areas, this being done without the developer attacking the base to any great extent.
The base bared in the non-image areas must have a great affinity for water, i.e. must be very hydrophilic, in order to take up water rapidly and permanently and to have a sufficiently repellant action toward the fatty printing ink as required in the lithographic printing process.
The adhesion of the photosensitive layer before exposure, and of the printing areas of the layer after exposure, must be adequate.
The base should possess good mechanical stability, for example to abrasion, and good chemical resistance, in particular to alkaline media.
A particularly frequently used starting material for such bases is aluminum, the surface of which is roughened by conventional methods, by drybrushing, wetbrushing, sand blasting, chemical treatment and/or electrochemical treatment. To increase the abrasion-resistant, electrochemically roughened substrates, in particular, are subjected to an anodizing step to build up a thin oxide layer. These anodic oxidation processes are usually carried out in electrolytes such as H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, H.sub.3 BO.sub.3, amidosulfonic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures of these. The oxide layers produced in these electrolytes or mixtures of electrolytes differ in structure, layer thickness and resistance to chemicals. In offset printing plate production in practice, in particular an aqueous H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4 solution is employed. With regard to H.sub.2 SO.sub.4 -containing electrolytes, reference may be made to, for example, U.S. Pat. No. 4,211,619 and the prior art mentioned therein.
Aluminum oxide layers produced in aqueous H.sub.2 SO.sub.4 -containing electrolytes are amorphous and, when used in offset printing plates, usually have a weight per unit area of about 0.5 to 10 g/m.sup.2, corresponding to a layer thickness of about 0.15 to 3.0 .mu.m. The disadvantage of using such an anodically oxidized base for offset printing plates is the face that the oxide layers produced in H.sub.2 SO.sub.4 electrolytes have a relatively low resistance to alkaline solutions as used to an increasing extent in, for example, the processing of presensitized offset printing plates, preferably in modern developer solutions for irradiated negative-working or, in particular, positive-working radiation-sensitive layers.
The anodic oxidation of aluminum in aqueous electrolytes containing phosphorus oxyacids or phosphates is likewise known per se:
U.S. Pat. No. 3,511,661 describes a process for the production of a lithographic printing plate, in which the aluminum base is oxidized anodically at a temperature of at least 17.degree. C. in an at least 10% strength aqueous H.sub.3 PO.sub.4 solution, until the aluminum oxide layer has a thickness of at least 50 nm.
U.S. Pat. No. 3,594,289 discloses a process in which a printing plate base made of aluminum is oxidized anodically in a 50% strength aqueous H.sub.3 PO.sub.4 solution at a current density of 0.5 to 2.0 A/dm.sup.2 and at a temperature of 15.degree. to 40.degree. C.
The process for the anodic oxidation of aluminum bases, in particular for printing plates, according to U.S. Pat. No. 3,836,437 is carried out in a 5 to 50% strength aqueous Na.sub.3 PO.sub.4 solution at a temperature of 20.degree. to 40.degree. C. and a current density of 0.8 to 3.0 A/dm.sup.2 and for a period of 3 to 10 minutes. The aluminum oxide layer thus produced should have a weight of 10 to 200 mg/m.sup.2.
The aqueous bath for the electrolytic treatment of aluminum which is to be coated subsequently with a water-soluble or water-dispersible sustance contains, according to U.S. Pat. No. 3,960,676, 5 to 45% of silicates, 1 to 2.5% of permanganates, or borates, phosphates, chromates, molybdates or vanadates in an amount from 1% to saturation.
British Pat. No. 1,587,260 discloses a base for printing plates which carries an oxide layer which is produced by anodic oxidation of aluminum in an aqueous solution of H.sub.3 PO.sub.3 or a mixture of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.3. The resulting relatively porous oxide layer is then covered with a second oxide film of the "barrier layer" type, which can be formed, for example, by anodic oxidation in aqueous solutions containing boric acid, tartaric acid or borates. Both the first stage (Example 3, 5 min) and the second stage (Example 3, 2 min) are carried out very slowly, and furthermore the second stage is carried out at a relatively high temperature (80.degree.).
It is true that an oxide layer produced in these electrolytes is often more resistant to alkaline media than is an oxide layer produced in an electrolyte based on H.sub.2 SO.sub.4 solution. This oxide layer while having some other advantages, such as a paler surface, better water/ink balance or less absorption of dyes ("staining") in the non-image areas), also possesses significant disadvantages. In a modern manufacturing line for the production of printing plate bases, it is possible, using voltages and residence times conforming to practice, to produce oxide layers having a weight per unit area of, for example, only up to about 1.5 g/m.sup.2, which corresponds to a layer thickness which, of course, provides less protection from mechanical abrasion than does a thicker oxide layer produced in an H.sub.2 SO.sub.4 electrolyte. Because of the relatively large pore volume and pore diameter of an oxide layer produced in H.sub.3 PO.sub.4, the mechanical stability of the oxide itself is lower; this results in a further loss with respect to abrasion-resistant.
Processes have also been disclosed which seek to combine the advantages of both electrolytes by employing a two-stage treatment procedure.
The process for the production of aluminum printing plate bases according to British Pat. No. 1,410,768 is carried out as follows: the aluminum is first oxidized anodically in an H.sub.2 SO.sub.4 -containing electrolyte. This oxide layer is thereafter treated in a 5 to 50 vol % aqueous H.sub.3 PO.sub.4 solution, in the absence of an electric current. The actual oxide layer should have a weight per unit area of 1 to 6 g/m.sup.2, this weight decreases significantly during immersion in the aqueous H.sub.3 PO.sub.4 solution, for example by about 2 to 3 g/m.sup.2 per minute of immersion time for an aqueous H.sub.3 PO.sub.4 solution. Electrochemical treatment in the H.sub.3 PO.sub.4 solution (Example 11) and the use of a mixed electrolyte consisting of H.sub.3 PO.sub.4 /H.sub.2 SO.sub.4 (Example 12) are also said to be possible, with loss of oxide layer occurring in these cases, too.
A two-stage electrochemical treatment, first in an electrolyte based on H.sub.2 SO.sub.4 and then in an electrolyte based on H.sub.3 PO.sub.4, is also described in U.S. Pat. No. 3,940,321. In the two-stage anodic oxidation or treatment procedure, the oxide layer built up in the H.sub.2 SO.sub.4 electrolyte once again is redissolved to an excessive extent in the H.sub.3 PO.sub.4 solution under the conventional conditions.
Occasionally, procedures for carrying out certain surface modifications even before the anodic oxidation in H.sub.2 SO.sub.4 solutions have also been described, for example:
U.S. Pat. No. 4,278,737 describes an electrolysis in a bath containing borate ions carried out prior to the anodic oxidation in a second bath (for example an aqueous H.sub.2 SO.sub.4 solution); the pH value of the first bath should be 9 to 11 and the treatment temperature 50.degree. to 80.degree. C., the thickness of the first layer should be at least 2 .mu.m, and that of the second layer should be greater (for example, about 20 .mu.m).
British Pat. No. 1,523,030 describes an electrolysis in an aqueous solution consisting of a salt (such as a borate or phosphate) and, if appropriate, an acid or a salt for producing a barrier layer (for example, boric acid or ammonium borate).
However, both publications relate only to aluminum which is intended to be used for window frames, panels (wainscots) and fixing components for building structures or decorative aluminum moldings for vehicles or domestic articles. Moreover, the formation of relatively thin layers would mean that these could become too easily detached again in the second treatment.
In British Pat. No. 1,412,929, an aluminum surface is treated with hot water or steam (with the formation of a boehmite layer), after which electrolysis is carried out in an aqueous solution of a salt of silicic, phosphoric, molybdic, vanadic, permanganic, stannic or tungstic acid. This treatment is intended to produce greater layer thickness, improved toughness, a finer structure and hence greater corrosion-resistance (for example, to acids or alkali). U.S. Pat. No. 3,945,899 also describes a similar process, wherein the surface of the aluminum can be, not only in the form of a boehmite layer, but also in the form of a chemically "modified layer", as the result of a chromate or phosphate treatment. In the Examples, the duration of electrolysis is from 2 to 10 minutes. However, both treatment steps are too protracted for modern manufacturing lines, and furthermore, the non-electrolytically produced aluminum layers do not conform very well to the practical requirements which high-performance printing plates have to meet (for example, in respect to the abrasion-resistance and the interactions with the photosensitive layer).
In European Pat. No. 0,007,233 and No. 0,007,234, aluminum bases for printing plates are anodically oxidized in such a way that they run, as an intermediate conductor, first through a bath containing aqueous 45% strength H.sub.3 PO.sub.4 and an anode, and then into a bath containing aqueous 15% strength H.sub.2 SO.sub.4 and a cathode. The two electrodes can also be connected to an a.c. voltage source. It is also stated (although no further specifications are given) that the treatment with H.sub.3 PO.sub.4 can be purely a dip treatment, and that it would also be possible to use neutral or alkaline solutions instead of the acids.
German Offenlegungsschrift No. 32 06 470 which has not been previously published and has an earlier priority date describes a two-stage oxidation process for the production of bases for offset printing plates in which the anodic oxidation is carried out in (a) an aqueous electrolyte based on sulfuric acid and (b) an aqueous electrolyte containing phosphoroxo, phosphorfluoro and/or phosphoroxofluoro anions. Another publication, German Offenlegungsschrift No. 33 12 497, which has not been published previously and has an earlier priority date likewise describes such a two-stage process. In this process, however, anodic oxidation is carried out first in an electrolyte containing phosphoric acid and only then in an electrolyte containing sulfuric acid.
British Pat. No. 20 88 901 discloses a two-stage anodic oxidation process for printing plate bases made of aluminum, wherein an aqueous electrolyte containing H.sub.3 PO.sub.4 is employed in the first stage, and an aqueous electrolyte containing H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4 is employed in the second stage. In the first stage, the solution employed contains at least 250 g of H.sub.3 PO.sub.4 per liter.
Monitoring a bath in the case of a mixed electrolyte is always difficult and expensive, so that the use of mixed electrolytes is avoided to the extent possible in modern manufacturing lines. The use of relatively high concentrations of H.sub.3 PO.sub.4 has, if anything, proved to be disadvantageous owing to the pronounced redissolution effects; this also applies to the use of a.c. current in the two-stage anodization. Connecting both electrodes to a current source can also be disadvantageous, since such a variant is more difficult to control from the point of view of production engineering.
Oxide layers produced initially in H.sub.3 PO.sub.4 -containing aqueous electrolytes are known to form a relatively compact barrier layer, which helps to increase the alkali-resistance of the oxide and hence to protect the aluminum underneath. However, in a subsequent treatment in H.sub.2 SO.sub.4 -containing aqueous electrolytes, a compact barrier layer of this type can often be, if anything, troublesome, since its electrical resistance first has to be overcome, and high voltages are therefore required. As a result, there is an increased danger of the occurrence of "burn-outs", i.e. penetration through the initially formed oxide layer. These burn-outs are unacceptable with regard to use in the lithography field.